(a) Field of the Invention
The present invention relates to a technology for mounting an electronic component such as a semiconductor element or the like on a wiring board. More particularly, it relates to an electronic component mounted structure in which a board serving as an interposer is interposed between a wiring board and an electronic component mounted thereon.
In such a mounted structure, the wiring board plays a role in mounting of an electronic component such as a semiconductor element or the like, and thus is also referred to as a “semiconductor package” or simply a “package” for convenience in the description below.
(b) Description of the Related Art
Flip chip bonding is one of methods for mounting an electronic component (chip) such as a semiconductor element on the surface of a wiring board (package). In mounting by the flip chip bonding, a chip and a package are electrically connected to each other by being bonded together with bumps interposed therebetween. Specifically, the bumps are formed on both of the chip side and the package side using the same metal material or different metal materials. Alternatively, metal bumps are formed on the chip side alone. An example of the bonding with the same metal includes bonding between a solder bump and another solder bump. Examples of the bonding with different metals include bonding between a copper (Cu) bump and a solder bump, bonding between a gold (Au) bump and a solder bump, and the like.
In any form of the bonding, the electrical connection between the chip and the package is achieved using at least a solder bump therebetween. Materials usable for the solder bump include a eutectic solder and a lead-free solder. The eutectic solder has a tin (Sn)-lead (Pb) composition. The lead-free solder is represented by Sn-silver (Ag) based solder, Sn—Cu based solder, Sn-zinc (Zn) based solder, or the like. Any of the solder materials contains tin (Sn) as the principal metal.
The description is continued taking a specific mounted structure as an example. A package is provided with a solder material (bump) formed on a pad (for example, a conductor layer made of copper (Cu)/nickel (Ni)/gold (Au)) exposed from the outermost insulating layer (typically, a solder resist layer) on the chip mounting surface side of the package. A chip to be mounted is provided with a projecting terminal (for example, Cu bump) formed on an electrode pad exposed from a protection film on the chip. The projecting terminal is brought into contact with the solder bump of the package. The solder bump is melted by reflowing, and thus the chip is electrically connected to the package (flip chip mounting). Further, an underfill resin (thermosetting resin) is filled into a gap between the mounted chip and the package, and is thermally cured to fix the chip on the package.
As an example of technique related to the above-mentioned prior art, Japanese unexamined Patent Publication (Kokai) 2002-151551 discloses a flip chip mounting structure. This flip chip mounting structure includes a semiconductor chip having an electrode and a noble metal (Au) bump formed on the electrode, and a wiring board having a surface provided with a connection terminal (Cu or the like). A thermosetting resin sheet is inserted between the semiconductor chip and the wiring board. Then, the resultant structure is pressed and heated while receiving ultrasonic vibration application. Thereby, the noble metal bump is connected to the connection terminal by metal bonding, and the chip is fixed to the wiring board with the thermally cured resin.
In the above-described conventional mounting technique by the flip chip bonding, the electrical connection between the chip and the package is achieved with the solder bump which contains at least tin (Sn) as the principal metal. This causes the following problems at the time of use (at the time of conducting a current) after the chip is mounted on the package.
Specifically, electromigration is easy to occur due to a current passing through a connection portion between the chip and the package, as the pitch between the terminals is further reduced because of miniaturization of packages and chips, higher integration (microfabrication) of wiring patterns, and the like. As a result, a void is produced in a portion of the connection portion, which increases the electrical resistance and reduces the bonding strength at the portion. In some cases, a breakage is caused at the portion (open circuit). Particularly, the electromigration occurs more remarkably where solder (alloy containing Sn) which is one of low-melting-point metals that readily cause electromigration is interposed between the chip and the package.
Namely, at the time of use (at the time of conducting a current) after the chip is mounted on the package, an electromigration phenomenon occurs between the solder (alloy containing Sn) which is the bump material, and a metal (Au, Ni, Cu, or the like) for the terminal. The phenomenon accumulatively causes problems such as an increase in the electrical resistance, an open circuit or the like at the connection portion, and thus lowers a reliability in the connection.